/* The Computer Language Benchmarks Game
 * http://benchmarksgame.alioth.debian.org/
 *
 * contributed by Christoph Bauer
 *
 */

#include <math.h>
#include <stdio.h>
#include <stdlib.h>

#define pi 3.141592653589793
#define solar_mass (4 * pi * pi)
#define days_per_year 365.24

struct planet {
    double x, y, z;
    double vx, vy, vz;
    double mass;
};

void advance(int nbodies, struct planet * bodies, double dt)
{
    int i, j;

    for (i = 0; i < nbodies; i++) {
        struct planet * b = &(bodies[i]);
        for (j = i + 1; j < nbodies; j++) {
            struct planet * b2 = &(bodies[j]);
            double dx = b->x - b2->x;
            double dy = b->y - b2->y;
            double dz = b->z - b2->z;
            double distance = sqrt(dx * dx + dy * dy + dz * dz);
            double mag = dt / (distance * distance * distance);
            b->vx -= dx * b2->mass * mag;
            b->vy -= dy * b2->mass * mag;
            b->vz -= dz * b2->mass * mag;
            b2->vx += dx * b->mass * mag;
            b2->vy += dy * b->mass * mag;
            b2->vz += dz * b->mass * mag;
        }
    }
    for (i = 0; i < nbodies; i++) {
        struct planet * b = &(bodies[i]);
        b->x += dt * b->vx;
        b->y += dt * b->vy;
        b->z += dt * b->vz;
    }
}

double energy(int nbodies, struct planet * bodies)
{
    double e;
    int i, j;

    e = 0.0;
    for (i = 0; i < nbodies; i++) {
        struct planet * b = &(bodies[i]);
        e += 0.5 * b->mass * (b->vx * b->vx + b->vy * b->vy + b->vz * b->vz);
        for (j = i + 1; j < nbodies; j++) {
            struct planet * b2 = &(bodies[j]);
            double dx = b->x - b2->x;
            double dy = b->y - b2->y;
            double dz = b->z - b2->z;
            double distance = sqrt(dx * dx + dy * dy + dz * dz);
            e -= (b->mass * b2->mass) / distance;
        }
    }
    return e;
}

void offset_momentum(int nbodies, struct planet * bodies)
{
    double px = 0.0, py = 0.0, pz = 0.0;
    int i;
    for (i = 0; i < nbodies; i++) {
        px += bodies[i].vx * bodies[i].mass;
        py += bodies[i].vy * bodies[i].mass;
        pz += bodies[i].vz * bodies[i].mass;
    }
    bodies[0].vx = - px / solar_mass;
    bodies[0].vy = - py / solar_mass;
    bodies[0].vz = - pz / solar_mass;
}

#define NBODIES 5
struct planet bodies[NBODIES] = {
    {                               /* sun */
        0, 0, 0, 0, 0, 0, solar_mass
    },
    {                               /* jupiter */
        4.84143144246472090e+00,
        -1.16032004402742839e+00,
        -1.03622044471123109e-01,
        1.66007664274403694e-03 * days_per_year,
        7.69901118419740425e-03 * days_per_year,
        -6.90460016972063023e-05 * days_per_year,
        9.54791938424326609e-04 * solar_mass
    },
    {                               /* saturn */
        8.34336671824457987e+00,
        4.12479856412430479e+00,
        -4.03523417114321381e-01,
        -2.76742510726862411e-03 * days_per_year,
        4.99852801234917238e-03 * days_per_year,
        2.30417297573763929e-05 * days_per_year,
        2.85885980666130812e-04 * solar_mass
    },
    {                               /* uranus */
        1.28943695621391310e+01,
        -1.51111514016986312e+01,
        -2.23307578892655734e-01,
        2.96460137564761618e-03 * days_per_year,
        2.37847173959480950e-03 * days_per_year,
        -2.96589568540237556e-05 * days_per_year,
        4.36624404335156298e-05 * solar_mass
    },
    {                               /* neptune */
        1.53796971148509165e+01,
        -2.59193146099879641e+01,
        1.79258772950371181e-01,
        2.68067772490389322e-03 * days_per_year,
        1.62824170038242295e-03 * days_per_year,
        -9.51592254519715870e-05 * days_per_year,
        5.15138902046611451e-05 * solar_mass
    }
};

double start() {
    offset_momentum(NBODIES, bodies);
    return energy(NBODIES, bodies);
}

double run(int n) {
    int i;
    for (i = 1; i <= n; i++)
        advance(NBODIES, bodies, 0.01);
    return energy(NBODIES, bodies);
}
